Above is a grating, or a grid. But there is something hidden in it. Can you guess? Or see it?
Try the button “Shake”, or, better shake your screen if possible, or your head. Do you see it now?
Another way to reveal the mystery image is to blur your view (screwing up your eyes) or moving away from the screen (thus using your eye optics’ low pass to reduce grating contrast).
You can play around with the width of the grating bars (“Period”), or their opacity (“Alpha”). With a very fine grating (period ≤ 5 pixels) the image becomes visible.
Or, cheating, switch off the grid with the check box.
The effect is constructed by adding a contrast-reduced image and a high-contrast grid, avoiding floor/ceiling effects.
Similar pictures float around a lot on the Internet. I have not found an original source yet, so I coined the term “grid masking” (for now, until someone tells me this is well known.) I chose that term because of my off-the-cuff hypothesis to explain this effect: The image signal is weak, and the grating with its high contrast swamps the detection channels, masking the weak image.
So why does shaking help? It causes motion of the bright and dark grid lines, effectively reducing their contrast. For the hidden image, motion blur has less effect because its structures are coarser.
Wikipedia: Visual Masking. “pattern masking” would be the case here